Forcefulness shown by transformed Green Party finally (6)
I believe the answer is:
Here is my best explanation:
'forcefulness' is the definition.
(energy is a kind of forcefulness)
'transformed green party finally' is the subsidiary indication.
'transformed' indicates anagramming the letters.
'finally' indicates one should take the final letters.
The last letter of 'party' is 'y'.
'green' with letters rearranged gives 'energ'.
'shown by' is the link.
(Other definitions for energy that I've seen before include "Power; capacity for work", "exertion", "Vitality, power", "Zip", "'Vigour, vitality (6)'", "Vitality, get-up-and-go", "Zeal", "Vim", "Power or vigorous activity" and "Zest".)